In the electrolytic refining and recovery of copper in an electrolytic cell using a suitable electrolyte, it is desirable to circulate the electrolyte through the internal space of the electrolytic cell in order to accelerate migration toward the cathode of copper ions in the electrolyte as a result of electrolysis and to carry out the electrolysis efficiently while maintaining uniformly the copper ion concentration in the electrolyte and maintaining constantly the temperature of the electrolyte.
In this process a current density in the order of 250 A/m.sup.2 has been considered to be an upper limit, due to the fact that the anode tends to be passivated and the effect of deposition of the metal on the cathode tends to be deteriorated when the current density exceeds the above limit. However, with the development of thyristors, it has become possible to easily control the flow of a large current and reverse the direction of such current flow. A so-called PRC (Periodic Reverse Current) method utilizing thyristors has been proposed in this field in which the direction of flow of a large current is periodically reversed. This PRC method is effective and advantageous among others as the current density in the electrolytic refining of copper can be greatly increased to improve productivity; the unit construction cost is relatively low; and the labor cost can also be reduced, although it has the drawback that the electrical power requirement is increased.
Thus, there are various problems to be solved for the successful production of highly pure copper by electrolysis at high current density utilizing the PRC method. To solve one of these problems, it is necessary to increase the amount of circulating electrolyte to a value greater than that circulated hitherto in order that the electrolytic refining at high current density can be successfully carried out.
In the electrolytic refining of copper according to conventional methods in an electrolytic cell, the amount of the circulating electrolyte is generally 20 to 25 l/min. However, an amount considerably greater than this value must be supplied to the electrolytic cell when the electrolytic refining of copper is carried out according to the PRC method. Insoluble impurities precipitate or settle as slime on the bottom of the electrolytic cell when the copper anode is progressively dissolved into the electrolyte as the electrolysis proceeds.
In order that electrolytic copper of good quality or high purity can be consistently produced by electrolysis with high current density obtained by the PRC method, it is essentially required to increase the circulating amount of the electrolyte without giving rise to undesirable floating movement of the slime settled on the bottom of the electrolytic cell. Meanwhile, an enlargement in the capacity of the electrolytic cell is also demanded together with the increase in the current density, and a novel method for circulating the electrolyte is demanded to deal with the increase in both the cell capacity and current density.
There are various methods for circulating electrolyte through an electrolytic cell used for the electrolytic refining of copper. According to the method conventionally employed in the art, the electrolyte is supplied from one side of the electrolytic cell and discharged from the other or opposite side of the cell. This method may be further classified into a plurality of methods. According to one of these methods, a supply port and a discharge port are provided respectively in the middle of the confronting side walls of the electrolytic cell for supply and discharge of the electrolyte into and out of the cell. In another method, a supply port and a discharge port are provided respectively at the diagonally opposite corners of the electrolytic cell for supply and discharge of the electrolyte into and out of the cell. However, these methods are undesirable in electrolysis at high current density which results in degrading the quality of electrolytic copper produced and reducing the rate of recovery of noble metals such as gold and silver. This is because such attempts to increase the amount of the circulating electrolyte results in undesirable floating of the slime settled on the bottom of the cell and suspension of the slime in the electrolyte. Further, when the capacity of the electrolytic cell is enlarged and the current density used for the electrolysis is also increased withough increasing the amount of the circulating electroyte, the concentration of copper in the upper layer of the internal space of the electrolytic cell differs greatly from that in the lower layer in the internal space of the cell, and the copper concentration in the lower layer becomes higher than that in the upper layer by about 7 to 8 g/l, due to the fact that the amount of the circulating electrolyte is insufficient compared with the cell capacity. On the other hand, a situation reverse to that of the copper concentration distribution occurs in the concentration distribution of free sulfuric acid. Consequently, the anode is frequently non-uniformly dissolved and this phenomenon makes impossible further continuation of electrolysis in the so-called passivated state due to non-uniform dissolution. This tendency becomes more predominant with increase in the current density, and finally, the electrolysis with increased current density will become impossible. The essential conditions required for successful production of electrolytic copper of good quality or high purity with an electrolytic cell of large capacity and high current density using a circulating electrolyte include:
1. capability of supplying sufficient copper ions and additives to the cathode surface; PA1 2. minimization of fluctuation in the concentration of the electrolyte in the cell; PA1 3. minimization of fluctuation in the temperature of the electrolyte in the cell; and PA1 4. elimination of floating or suspension of slime in the electrolyte caused by the circulating flow;
Applicants have succeeded in developing a highly efficient electrolytic cell of large capacity which satisfies the above conditions and which is capable of stable operation with high current density for the electrolytic production of highly pure copper.